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Insects are the only invertebrates that can fly.  Their wings develop as evaginations of the exoskeleton during morphogenesis but they become fully functional only during the adult stage of an insect's life cycle.  The wings may be membranous, parchment-like, heavily sclerotized, fringed with long hairs, or covered with scales.  Most insects have two pairs of wings -- one pair on the mesothorax and one pair on the metathorax (never on the prothorax).  Wings serve not only as organs of flight, but also may be adapted variously as protective covers (Coleoptera and Dermaptera), thermal collectors (Lepidoptera), gyroscopic stabilizers (Diptera), sound producers (Orthoptera), or visual cues for species recognition and sexual contact (Lepidoptera).

In most cases, a characteristic network of veins runs throughout the wing tissue.  These veins are extensions of the body's circulatory system.  They are filled with hemolymph and contain a tracheal tube and a nerve.  In membranous wings, the veins provide strength and reinforcement during flight.  Wing shape, texture, and venation are quite distinctive among the insect taxa and therefore highly useful as aides for identification.

Wing adapations and modifications:

Characteristic Appearance Order(s)
Elytra -- hard, sclerotized front wings that serve as protective covers for membranous hind wings
Hemelytra -- front wings that are leathery or parchment-like at the base and membranous near the tip
Hemiptera: Heteroptera
Tegmina -- front wings that are completely leathery or parchment-like in texture
and Mantodea
Halteres -- small, club-like hind wings that serve as gyroscopic stabilizers during flight
Fringed wings -- slender front and hind wings with long fringes of hair
Hairy wings -- front and hind wings clothed with setae
Scaly wings -- front and hind wings covered with flattened setae (scales)
Hamuli -- tiny hooks on hind wing that hold front and hind wings together
Frenulum -- Bristle near base of hind wing that holds front and hind wings together

Wing Venation:

The archedictyon is the name given to a hypothetical scheme of wing venation proposed for the very first winged insect.  It is based on a combination of speculation and fossil data.  Since all winged insects are believed to have evolved from a common ancestor, the archediction represents the "template" that has been modified (and streamlined) by natural selection for 200 million years.  According to current dogma, the archedictyon contained 6-8 longitudinal veins.  These veins (and their branches) are named according to a system devised by John Comstock and George Needham -- the Comstock-Needham System:

Costa (C) -- the leading edge of the wing
Subcosta (Sc) -- second longitudinal vein (behind the costa), typically unbranched
Radius (R) -- third longitudinal vein, one to five branches reach the wing margin
Media (M) -- fourth longitudinal vein, one to four branches reach the wing margin
Cubitus (Cu) -- fifth longitudinal vein, one to three branches reach the wing margin
Anal veins (A1, A2, A3) -- unbranched veins behind the cubitus

Names of crossveins are based on their position relative to longitudinal veins:

c-sc crossveins run between the costa and subcosta
r crossveins run bewteen adjacent branches of the radius
r-m crossveins run between the radius and media
m-cu crossveins run between the media and cubitus

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